Field of the Art
The present invention relates in general to an elastic mounting structure disposed between a body of a vehicle and a power unit for mounting the power unit on the vehicle body, and more particularly to a fluid-filled elastic vibration-damping structure which is capable of selectively exhibiting high vibration-damping characteristics and high vibration-isolating characteristics, depending upon the conditions of the vehicle.
In the prior art of installing a power unit (consisting of an engine, a transmission and other components as a unit) to a rigid support member of a vehicle, there has been known an elastic or resilient mounting structure, which is interposed between the power unit and the rigid member of the vehicle so as to support the power unit, while insulating or isolating operational vibrations of the engine from the vehicle body, and damping or attenuating vibrations of the vehicle body and the engine from the road surface while the vehicle is running.
Such mounting structure is generally required to exhibit vibration-damping characteristics for damping low-frequency vibrations (of lower than 10 Hz) of the vehicle body during running of the vehicle on bumpy road surfaces, and vibration-isolating characteristics for isolating or insulating vibrations of comparatively higher frequencies (of higher than several tens of Hz) of the power unit. In a conventional mounting structure using an elastic isolator member disposed between two rigid members, the above-indicated two different characteristics are not available. If the elastic member is made of a rubber material which has a high damping effect, its spring rate (or constant) is too high to effectively isolate the vibrations in a relatively high frequency range. On the other hand, if the elastic member is formed of a rubber material having a relatively high spring rate to provide a comparatively high vibration-isolating capability, its loss factor is reduced and its vibration-damping capability is accordingly reduced.
In particular, the mounting structure is required to have a high dynamic rigidity and exhibit high vibration-damping characteristics for attenuating vibrations of the power unit of relatively high amplitude, when the engine undergoes shaking vibrations (resonance of the engine mass-mounting spring system) during running of the vehicle, or when the engine torque is rapidly changed upon a sudden start or acceleration of the engine. On the other hand, the mounting structure is required to have a low dynamic rigidity and a low dynamic spring rate for isolating the vibrations of the power unit from the vehicle body while the engine is idling or while the vehicle is running at a high speed. However, none of the known mounting structures have been found to satisfy these two different needs.
In the meantime, various fluid-filled elastic bushings have been proposed, which are adapted to damp or absorb vibrations by means of a resistance to flow of a fluid through a suitable orifice, as well as elastic deformation of an elastic member. Examples of such fluid-filled elastic bushings are disclosed in Japanese Patent Applications which were laid open in 1978 and 1982 under Publication Nos. 53-5376 and 57-9340, respectively. These fluid-filled bushings have an orifice which permits restricted flows of a fluid between two fluid chambers, with a predetermined resistance to flows of the fluid through the orifice, thereby enabling the bushings to perform a vibration-damping function.
Although such a fluid-filled elastic bushing is effective for damping low-frequency vibrations of high amplitude such as shaking vibrations of an engine, the bushing is not satisfactory for isolation or insulation of idling vibrations of the engine in a frequency range of about 5-40 Hz which overlaps the frequency range of the shaking vibrations. More specifically, while the engine is idling with the vehicle stopped, it is more important to prevent the idling vibrations of the engine due to fluctuation of its torque, from being transmitted to the vehicle body, rather than to damp or attenuate such idling vibrations. To this end, the elastic bushing must exhibit a low dynamic spring rate, i.e., soft spring characteristics. However, the restriction of the fluid flow by the orifice will increase the dynamic rigidity of the fluid-filled bushing. In other words, the high-damping characteristics for the low-frequency shaking vibrations of the engine is incompatible with the soft spring characteristics for the idling vibrations of the engine which fall in the same low frequency range.